A gas turbine engine conventionally includes a compressor for compressing ambient air for being mixed with fuel and ignited to generate combustion gases in a combustor. A turbine receives the hot combustion gases and extracts energy therefrom for powering the compressor and producing output power, for example for powering an electrical generator. The turbine conventionally includes one or more stages of stator nozzles or vanes, rotor blades and annular shrouds around the turbine blades for maintaining appropriate clearances therewith. As the turbine inlet temperatures have increased to improve the efficiency of gas turbine engines, it has become necessary to provide a cooling fluid, such as air, to the turbine vanes, blades and shrouds to maintain the temperatures of those components at levels that can be withstood by the materials thereof, to ensure a satisfactory useful life of the components. Cooling is typically accomplished by extracting a portion of the air compressed by the compressor from the compressor and conducting it to the components of the turbine to cool the same. Any air compressed in the compressor and not used in generating combustion gases necessarily reduces the efficiency of the engine. Therefore, it is desirable to minimize the amount of cooling air bled from the compressor.
Turbo-machinery performance and reliability are impacted by the clearances between rotating and static hardware. Tighter clearances produce higher efficiencies, but also increase the likelihood of damage from rubs. During operation, the casing of the gas turbine cools off much faster than the rotor on a typical turbine rotor. During a warm or hot restart, the thermal mismatch between the casing and the rotor may cause the rotor to have a greater initial component of thermal growth than the stator and then, as the unit increases in speed, the rotor experiences an additional component of mechanical growth. This causes a transient clearance pinch point. As time progresses and the stator heats up, the casing grows away from the rotor and results in more open full speed full load (FSFL) clearances. The build clearances of a unit must be set in such a way as to avoid a rub during the transient pinch point and still be tight at FSFL. The difference in minimum clearance to FSFL clearance is defined as “entitlement.” The entitlement is determined by the thermal mismatch between rotor and casing.
Previous attempts to address this problem have included active clearance control systems. For example, an inner turbine shell may be heated with a medium (e.g. air, N2, steam) during startup to grow the stator away from the rotor or to be cooled at FSFL to bring the shell closer to the rotor. As another example, a hydraulic ram may be used to move the rotor axially into position after the unit has reached FSFL. The angle of the bucket tips and the casing shrouds in the turbine are greater than the associated angle and the compressor and this angle mismatch enables the elimination of rub between the bucket tips and the casing shrouds during the transient pinch point.
The prior attempts to avoid a rub during the transient pinch point require relatively large clearances between the buckets and the casing and/or the use of an expensive system to be continuously run to achieve clearances during operation of the gas turbine at FSFL.